1. Field of the Invention
The present invention relates to a structure for improving the performance of various optical guided-wave devices using an optical wave guide for such applications as optical power modulation, optical switching, plane of polarization control, optical phase matching and propagation mode control, and further relates to the manufacturing method of said structure. 2. Description of the Prior Art
Conventional optical guided-wave devices such as optical modulators, optical switches, plane of polarization control devices, optical phase matching and optical propagation mode control devices form a single propagation mode optical wave guide in a dielectric single crystal having an electro-optic effect (such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3)), and control the passage of light through the optical wave guide by manipulating the shape of the optical wave guide, providing appropriately shaped electrodes, and utilizing the electrooptic effect. The structure of such optical guided-wave devices is described in Waveguide Electrooptic Modulators by R. Alferness, (IEEE Transactions on Microwave and Techniques, Vol. MTT-30, No. 8, 1121-1137 (August, 1989)). Manufacturing methods for optical wave guides are likewise described in Optical Waveguide Modulators by I. Kaminow (IEEE Transactions on Microwave and Techniques, Vol. MTT-23, No. 1, 57-70 (1975)).
In one such manufacturing method, lithium niobate or lithium tantalate is heat treated at a high temperature to modify the refractive index of the material by out-diffusing the lithium. Alternatively, a metallic film of, for example, titanium is formed by vapor deposition and thermally diffused at a high temperature to raise the refractive index of the diffused area slightly above that of the surrounding area. In either case, the difference in refractive indexes is used to trap light.
An example of a Mach-Zehnder type optical modulator using a titanium diffusion is described in Japanese patent laid-open publication SHO 63-261219. In another method described in the literature, a metallic mask is formed over the specified areas and a proton-ion exchange is induced in phosphoric acid at 200.degree. C. to 300.degree. C., partially modifying the refractive index and forming the optical wave guide. Manufacturing methods that rely on out-diffusion, thermal diffusion, or ion exchange from the surface all form the optical wave guide by means of diffusion from the surface. The cross section of the optical wave guide is therefore necessarily determined by the diffusion process, resulting in numerous problems.
One of the biggest problems is coupling loss between the optical wave guide and the optical fiber. While the cross section of an optical fiber is circular, the shape of most conventional optical wave guides is roughly an inverted triangle due to formation of the optical wave guide by diffusion from the surface. Because the strength of the guided light is greatest near the surface, optical coupling with the optical fiber is poor, resulting in significant loss. Reducing this optical coupling loss is therefore an extremely important topic in optical guided-wave device design.
Another problem caused by diffusion processing is greater optical propagation loss after diffusion processing than before. With a titanium diffusion optical wave guide, for example, propagation loss of approximately 1 dB/cm normally occurs. Reducing propagation loss is therefore another major topic in optical guided-wave device design.
A third problem is the increase in optical damage resulting from diffusion processing. Optical damage refers to the increase in propagation loss over time when a high intensity light source or a short wavelength light source is input to a diffusion-type optical wave guide. This is believed to be caused by the diffusion of ions in the optical wave guide resulting in increased trapping of electrons in the optical wave guide.
It should be noted that methods for forming an optical wave guide without relying on diffusion processing have been described. One of these is described by Kaminow (see above reference). In this method, lithium niobate crystals are grown on top of a lithium tantalate layer, or a lithium niobate thin-film is formed by sputtering on top of a lithium niobate or lithium tantalate layer, and the optical wave guide is formed in this lithium niobate top layer. A similar method is described in Japanese patent laid-open publication SHO 52-23355. This method also forms an epitaxial growth lithium niobate top layer over a substrate of lithium tantalate (e.g.) using liquid phase, gas phase, fusion, or other method, and forms the optical wave guide in this top layer. There are, however, several problems with these optical wave guide formation methods using such thin-film crystal growth technologies. First, it is extremely difficult to achieve a thickness of greater than 5 .mu.m in epitaxial growth films, and productivity is accordingly poor, because of the growth speed and flaws occurring in the crystals while being grown. In addition, the coupling characteristics of a thin-film less than 5 .mu.m thick with an optical fiber having a core diameter of approximately 10 .mu.m are also poor. (The fiber core being where the light is trapped.)
Productivity is further hampered because a good quality single crystal thin-film cannot be obtained unless the lattice constants of the thin film is essentially same as those of the substrate. It is therefore extremely difficult to form a good lithium niobate thin-film on a lithium tantalate substrate, and a mixed niobium-tantalum crystal film is often used. Pure lithium niobate, however, offers superior overall optical wave guide characteristics when compared with a mixed crystal film.
While epitaxial growth of like materials is possible, the crystal orientation of the two layers will be the same, making it difficult to obtain an effective difference between the refractive index of the base substrate and that of the grown thin-film. This results in a solid substrate in which the optical wave guide cannot be formed.
If the thin-film formed by these thin-film growth technologies is not good, propagation loss will increase and optical damage will increase even when the layers are stacked thickly, and the resulting film is therefore not desirable.